1. Field of the Invention
The invention relates to monolithically integratable control circuits for switching inductive loads and, more particularly, to control circuits of the kind comprising a Darlington-type final stage for use in actuating relays, solenoids and d.c. motors.
2. Description of the Related Art
Switching control circuits of the kind described herein usually comprise a final power transistor coupled in series with the inductive load between the two terminals of a supply voltage generataor and alternately switched, via a base control signal, from a high-voltage low-current state to a low-voltage high-current state.
In the first state (the "off" state), the transistor is virtually an open circuit between the emitter and collector terminals. In the second or "on" state, a short-circuit is established. Thus, the two states respectively prevent or allow current to flow through the load.
As is known, the manner of operation of a transistor closest to the operation of an ideal switch is that in which the transistor operates in a saturation condition when closed and is in a cut off condition when open.
In the latter case, however, the maximum possible switching frequency of the transistor is mainly limited, during the change-over from a saturation condition to a cut-off condition by the effects of the storage of charge occurring during the conduction phase. The switching frequency limitation arises because the collector region of power transistors, which is dense and has high resistivity in order to withstand high inverse voltages, has a relatively long switching-off transient phase during which an increase in collector-emitter voltage does not correspond to a decrease in the collector current, the collector current remaining constant for a certain time.
This phase, of course, is the phase in which the transistor dissipates most energy even though it is of no use in operation. When the load coupled to the final stage is inductive, the counter-EMF induced by the variation in the current through the load, due to switching of the final stage, abruptly increases the collector-emitter voltage of the still conducting transistor during the switching-off phase. The counter-EMF, in combination with the supply voltage, thus produces very high power dissipation in the transistor, sometimes with destructive effect.
A reduction in the switching-off time, therefore, is advantageous both for increasing the maximum possible switching frequency and for improving the efficiency of the control circuit with regard to energy consumption, because of the reduction in time when the operation of the final power transistor departs from the operation of an ideal switch.
The usual method for reducing the switching time of a power transistor operating in saturation during the conduction stage is to couple the base of the transistor to a circuit means having a low impedance, thus enabling the charge stored in the transistor to flow away rapidly when the saturated transistor is switched off.
The circuit means can simply be a transistor which is actuated in phase opposition relative to the transistor to be switched off. The circuit means produces a current for extracting the charge from the base of the switched-off transistor. The phase-opposition transistor is inserted with its collector and emitter terminals between the load and the base of the power transistor to be switched off, or between the base and the supply voltage generator terminal to which the load is coupled.
In the first case the efficiency with which charge is extracted is not very high because the collector-emitter voltage applied to the extraction transistor is limited. Even so, extraction continues until switching-off is complete.
In the second case, however, the extraction transistor initially acts more efficiently since the collector-emitter voltage applied to it is higher. Extraction, however, is interrupted before the final transistor has been completely switched off, if the load is of the inductive type. The reason for interruption of extraction is that, during switching-off, a counter-EMF is induced in an inductive load that lowers the potential levels of the final transistor, coupled to the load, below the potential level of the negative supply terminal. Consequently, if the extracting transistor has its emitter coupled to the negative supply terminal, the extraction transistor is inversely polarized and stops extracting charges. On the contrary, a diode has to be inserted between the two transistors to prevent any feedback of current.
In order therefore to obtain high switching rates in a circuit for controlling inductive loads, it is necessary to combine the two previously-mentioned systems, using two different extraction transistors, the emitter of one transistor being coupled to the negative supply terminal, whereas the other transistor is coupled to the output terminal, as described e.g. in Italian Patent Application No. 20213/A 82 by the present applicants.
This method of extracting charge from the switched-off saturated transistor is very effective initially and continues until switching-off is complete.
Clearly, however, this method involves a more complicated circuit, owing to the polarization and control means required, and more space is needed for integration of the circuit elements, thus increasing implementation expense. The same considerations also apply to control circuits which the final power transistor is kept in the active zone of its operating range, but is switched by a transistor which operates in saturation when conductive. In that case, to increase the rate of switching, the base of the last-mentioned transistor must be coupled to one or two extraction transistors as previously described, so as to accelerate the discharge process.
This case applies, e.g., to the case of the control switching circuits to which the invention relates, such circuits comprising a Darlington circuit final stage made up of a final power transistor operating in the active zone and its control transistor operating in saturation, the two being interconnected at a common collector. NPN type transistors are normally used, owing to their switching characteristics.
Switching control circuits of the aforementioned kind are used for special applications in which it is important to reduce energy consumption by the circuit when inoperative, since such consumption is the largest item in the total consumption during the various operating stages. A Darlington circuit final stage has low consumption when inoperative, less than that of other final stages, since its current gain is very high.
Even though a Darlington circuit stage requires a minimum voltage for operation, equal to a base-emitter voltage plus a collector-emitter voltage at saturation, thus resulting in a greater loss of useful voltage, this voltage loss is an unimportant percentage of the supply voltages normally used for control circuits for switching inductive loads.
Furthermore, a Darlington circuit final stage, particularly if made up of NPN transistors, has considerable advantages regarding integration and can be switched more quickly than a single final transistor of equal power operating in saturation.